Suppress log spam from multiple reports of SIGQUIT shutdown.

When the postmaster sends SIGQUIT to its children, there's no real
need for all the children to log that fact; the postmaster already
made a log entry about it, so adding perhaps dozens or hundreds of
child-process log entries adds nothing of value.  So, let's introduce
a new ereport level to specify "WARNING, but never send to log" and
use that for these messages.

Such a change wouldn't have been desirable before commit 7e784d1dc,
because if someone manually SIGQUIT's a backend, we *do* want to log
that.  But now we can tell the difference between a signal that was
issued by the postmaster and one that was not with reasonable
certainty.

While we're here, also clear error_context_stack before ereport'ing,
to prevent error callbacks from being invoked in the signal-handler
context.  This should reduce the odds of getting hung up while trying
to notify the client.

Per a suggestion from Andres Freund.

Discussion: https://postgr.es/m/20201225230331.hru3u6obyy6j53tk@alap3.anarazel.de

src/backend/tcop/postgres.c

@@ -2789,6 +2789,18 @@ quickdie(SIGNAL_ARGS)
 	 * wrong, so there's not much to lose.  Assuming the postmaster is still
 	 * running, it will SIGKILL us soon if we get stuck for some reason.
 	 *
+	 * One thing we can do to make this a tad safer is to clear the error
+	 * context stack, so that context callbacks are not called.  That's a lot
+	 * less code that could be reached here, and the context info is unlikely
+	 * to be very relevant to a SIGQUIT report anyway.
+	 */
+	error_context_stack = NULL;
+
+	/*
+	 * When responding to a postmaster-issued signal, we send the message only
+	 * to the client; sending to the server log just creates log spam, plus
+	 * it's more code that we need to hope will work in a signal handler.
+	 *
 	 * Ideally these should be ereport(FATAL), but then we'd not get control
 	 * back to force the correct type of process exit.
 	 */
@@ -2802,7 +2814,7 @@ quickdie(SIGNAL_ARGS)
 			break;
 		case PMQUIT_FOR_CRASH:
 			/* A crash-and-restart cycle is in progress */
-			ereport(WARNING,
+			ereport(WARNING_CLIENT_ONLY,
 					(errcode(ERRCODE_CRASH_SHUTDOWN),
 					 errmsg("terminating connection because of crash of another server process"),
 					 errdetail("The postmaster has commanded this server process to roll back"
@@ -2814,7 +2826,7 @@ quickdie(SIGNAL_ARGS)
 			break;
 		case PMQUIT_FOR_STOP:
 			/* Immediate-mode stop */
-			ereport(WARNING,
+			ereport(WARNING_CLIENT_ONLY,
 					(errcode(ERRCODE_ADMIN_SHUTDOWN),
 					 errmsg("terminating connection due to immediate shutdown command")));
 			break;